One commuter said the service was the worst it had ever been
Campaigners have set up an action group to address what they say is an "all time low" on Southern rail services.
According to Sussex Rail Action, only 70% of Southern trains ran on time in January, and they have dubbed the service the UK's "new misery line".
Southern said it was "acutely aware that its recent train punctuality has not been as good as it should be", and apologised.
It said the figures quoted by the group were for the Sussex coast only.
The rail firm runs trains to Sussex and Surrey.
One of the commuters to have joined the action group, Ken Macdonald, said: "I've been commuting for 18 years and this is the worst it's ever been."
And fellow passenger Alex Ross added: "Other routes may be bad, but this is the new misery line for commuters."
The group, launched on Wednesday, is planning a series of events to increase pressure on Southern and other rail firms, and is setting up a website where passengers can share travel experiences.
Campaigners also said they wanted to ensure the government did not renew Southern's franchise.
In a statement, Southern said its Sussex coast performance had been "particularly poor", but that 90% of its trains had arrived on time over the past 12 months.
It said its re-conditioned trains had suffered from reliability problems leading to cancellations and delays that affected the whole Brighton Main Line.
It added: "Other problems faced by Southern and its passengers were high incidences of rail infrastructure failures such as problems with points, signalling and broken rails.
"Each of these failures meant that train services all over the Southern network were being disrupted on a daily basis.
"Coupled with this, the recent bad weather conditions only added to the problems and incidences such as passenger interventions, trespass on the lines and suicides did not help us to provide a punctual service."
Southern said its trains were now more reliable, work was taking place on the rail infrastructure, and there had been a full review of internal procedures to identify where improvements could be made.